A neuronal projection has been neuroanatomically identified to originate from the median raphe (MR) nucleus and to terminate in the hypothalamic suprachiasmatic (SCh) nuclei. These neurons have been further identified neurochemically to contain serotonin and to release this monoamine at their suprachiasmatic terminals. Stimulation and lesion studies at the level of the MR nucleus have suggested that this MR-SCh projection inhibited luteinizing hormone (LH) release and ovulation and may be involved in regulating other anterior and/or posterior pituitary hormones. The overall objective of the proposed research is to establish the nature of MR-SCh inputs to hypothalamic-pituitary function. We will manipulate the activity of this system by stimulating or lesioning the MR nucleus using stereotaxic procedures. Responses in various endocrine situations will be monitored by direct measurement of hypothalamic serotonin or LH-releasing factor, and by radioimmunoassay of plasma hormones (LH, follicle stimulating hormone and prolactin).